The basic operating mode of a DDD/AAI cardiac pacemaker is an AAI mode, with a single chamber atrial stimulation and a monitoring (detection) of the ventricular activity. This mode is maintained as long as atrio-ventricular conduction is normal, i.e., as long as each atrial event (atrial detection, corresponding to a spontaneous activity, or atrial stimulation corresponding to a paced event) is followed by an associated ventricular detection. In certain circumstances there exist atrio-ventricular blocks (AVB) involving a temporary defect of the depolarization of the ventricle. In this case, the cardiac pacemaker switches automatically to operate in a DDD mode, with parameters optimized for a temporary AVB situation. After the disappearance of the AVB, and thus a re-establishment of spontaneous atrio-ventricular conduction, provided that a certain number of conditions are fulfilled, the cardiac pacemaker switches automatically to operate in the AAI mode. This automatic mode switching between DDD and AAI is described, for example, in EP-A-0 488 904 and its counterpart U.S. Pat. No. 5,318,594 commonly assigned herewith to ELA Médical, Montrouge, France.
The starting point of the present invention arises from certain observations made by the inventors at the time of a clinical follow-up of patients equipped with AAI or DDD/AAI devices that have automatic mode switching, when these patients presented ventricular extrasystolies (VES). VES are spontaneous ventricular events that are not preceded by an associated atrial depolarization and present a significant shortening of the ventricular interval RR (or VR) compared to the preceding ventricular event. According to the coupling of VES as compared to the preceding ventricular event, the VES can occur either simultaneously with the atrial depolarization of the following beat (i.e., the cardiac cycle), or at a moment close to the end of the atrial escape interval (AEI), i.e., at the moment when an atrial stimulation must be delivered in the absence of a spontaneous event detected in the atrium.
It was appreciated that, in a certain number of situations that will be described in more detail below, the known devices present limits in their capacity to detect or manage correctly the cardiac electric activity in the event that a VES has occurred at certain critical moments of the cardiac wave. This incapacity to manage correctly all situations that are likely to occur in the event of the occurrence of VES causes:    1. a delivery to the heart of inappropriate atrial or ventricular stimulations, that are therefore ineffective—and potentially disadvantagous—to the hemodynamic flow;    2. with these AAI/DDD devices having automatic mode switching, accidental switching to the DDD mode because of a false diagnosis of an atrio-ventricular block (the device believes that it has detected an AVB, whereas a spontaneous conduction is actually present); and    3. a combination of the above two erroneous actions, i.e., an inappropriate stimulation followed by an unnecessary switching to the DDD mode.These inappropriate actions can create among certain vulnerable patients conditions that are favourable to the appearance of disorders of the ventricular rate/rhythm, which of course it is desirable to prevent.